1. Field of the Invention
The present invention concerns a helical antenna for broadcasting and receiving radio waves, especially waves in the meter band for radio signals in the frequency modulation audio broadcast band in the range 87 MHz to 110 MHz, and more generally in a frequency band from approximately 70 MHz to approximately 150 MHz.
2. Description of the Prior Art
At present an emit/receive antenna operating in the above frequency band is a "whip" or "wire" antenna which is pratically omnidirectional in azimuth and consists of a substantially vertical conductive wire have a length equal to one half-wavelength, i.e. approximately one to two meters.
To improve the broadcasting of stereo radio programs and to improve listening conditions for listeners along highways, emitters synchronized to the same frequency are installed alongside the highways. The emitters can also broadcast data signals to tune receivers automatically by means of digital messages identifying radio programs. One frequency modulation radio broadcast system of this kind is known as the Radio Data System (RDS). In the context of broadcasting digital information, the information can be produced by microcomputers for broadcast to receivers associated with remote-display panels in towns and in public transport vehicles, in parking lots, etc.
For all the above applications the receive or emit/receive antenna should be small for reasons of overall size in situ and esthetics, whilst retaining the omnidirectional character in a horizontal plane inherent to audio broadcasting. To satisfy the above conditions the antenna must be short, but this is to the detriment of bandwidth, which is reduced commensurately. In this case the antenna will be tuned to a relatively narrow bandwidth of a few megahertz.
To overcome dimensional adaptation problems for the whip antenna, the latter could be replaced by a simple helical antenna with a length of only approximately 20 centimeters, i.e. a length saving in the order of 75%. By selecting the number of turns of the antenna helix by trial and error, the antenna can be tuned to the required frequency and matched to the impedance of a coaxial line to which it is connected.
An antenna of the above kind can be fixed to the top of a bus stop post or to the top of an even higher tubular post for a parking lot signposting system or to an urban remote-display screen frame, which can be more than five meters above the ground. Access is difficult to all of the above antennas.
If a working of a private or local radio station decides to change the emission frequency from the station or is replaced by another working all the receive and/or emit/receive antennas must be tuned to the new emission frequency of the station which involves changing the antennas and carrying out new measurements to tune them.
A helical antenna having four parallel conductive helices extending about a common central axis in a same direction is disclosed in U.S. Pat. No. 5,489,916. A single dielectric helix concentric with the common axis lies within the four helices, and has a length substantially equal to that of the conductive helices, which are uniformly spaced from each other and fixed to the dielectric helix. A casing contains the helices and is secured to one end of the dielectric helix. A tuning device is secured to the other end of the dielectric helix and can turn relative to the housing so that rotating the tuning device modifies the pitch of the dielectric helix and consequently the common pitch of the conductive helices without significantly varying the nominal diameter of the helices. For example, if the number of turns per unit length decreases, the length of the conductive helices increases.
Another helical antenna with four helical radiating wires or "filaments" is disclosed in PCT WIPO International patent application WO 96/19846 and also comprises means for rotating one end of the wires relative to the fixed other end thereof in order to modify the length and pitch of the fils.
Antennas of the above kind are receive antennas for a satellite telecommunication system and radiate axially with circular polarization at very high frequencies above 1 GHz. Modifying the geometry of the conductive helices modifies the aperture of the radiation diagram of the antenna in radiation directions close to the vertical axial direction.